This invention relates to methods and apparatus for measuring and monitoring concentrations of dissolved radionuclides in fluid samples.
Radioactive elements (i.e., radionuclides), such as natural uranium (U.sup.235 /U.sup.238 /U.sup.234) and man-made uranium (U.sup.237) decay and emit alpha and beta particles having, respectively, energies between 4-10 MeV and 0.1-1.0 MeV. The emitted particles can be detected in air, water, soil, or other samples to determine the level of radioactivity. In particular, samples taken from sites near weapons and fuel production facilities may contain concentrations of alpha-emitting radionuclides in excess of permitted levels.
Uranium is rated by the U.S. Environmental Protection Agency (EPA) as being a "class-A carcinogen" at very low levels. In addition, this element has a high chemical toxicity; the EPA-proposed maximum concentration limit (MCL) for uranium in public drinking water supplies is 13.5 pCi/l (about 20 ppb). Uranium present in this concentration in 1 liter of water emits about 30 alpha particles each minute.
Currently, process, surface and ground waters at contaminated sites are monitored for alpha-emitters and other contaminants by intermittent sampling. These samples are chemically preserved by the addition of acid, entered into a chain of custody infrastructure, packaged for shipment and then sent to a central laboratory for analysis. Because the permitted levels of radionuclides are low, analysis in the laboratory usually involves a lengthy and costly procedure for separating the radionuclides from the water sample, concentrating them to form a thin layer, and then measuring the emission from the layer. For example, during a typical analytical testing procedure, the alpha-emitting radionuclides are separated from a water sample either by precipitation or evaporation; the separated radionuclides are then plated on a planchet (i.e., a glass or metal surface) and the emission is counted in vacuum using a biased silicon detector. The detected signals are then analyzed to determine the radionuclide concentration, which is then reported to the requester.
Because the sample analysis is typically done off-site at a remote laboratory, the current radionuclide-monitoring procedure has several shortcomings. For example, large variations in radionuclide concentrations at the site can go undetected due to the sporadic sampling necessarily involved with the collection/analysis procedure. Due to the high cost of the procedure, only limited numbers of samples are typically taken. In addition, the analysis may be inaccurate because of the time-dependent concentrations of radionuclides at the site; the amount of radionuclides detected in the off-site laboratory may not necessarily be representative of the current on-site concentrations. The current method is prone to errors because of the many handling steps involved. The current method is also unsuitable for process control because of the long time delay between collection and analysis.